Method and system for generating a broadband spectral continuum and continuous wave-generating system utilizing same

ABSTRACT

Method and system are disclosed for stable, multi-wavelength continuous wave (CW) generation using fiber-based supercontinuum and spectrum-slicing of its longitudinal modes. The continuum generated is coherent and stable, making it an attractive alternative as a spectrally-sliced source for continuous, multiple wavelength channels. A 140 nm wide supercontinuum with a 10 GHz repetition rate is generated in &lt;30 meters of fiber. To obtain CW channels with 40 GHz spacing, time-domain multiplexing and longitudinal mode slicing are utilized. To obtain stable, continuous wave operation, short-fiber supercontinuum generation and a pulse interleaving method are utilized. The invention may be utilized as a broadband wavelength-division multiplexed source.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of 09/454,106 filed Dec. 3, 1999, now U.S. Pat.No. 6,381,391 which is a division of 09/253,852 filed on Feb. 19, 1999,now U.S. Pat. No. 6,480,656 and entitled “Method and System forGenerating a Broadband Spectral Continuum, Method of Making the Systemand Pulse-Generating System Utilizing Same”.

GOVERNMENT RIGHTS

This invention was made with government support under AFOSR GrantF30602-97-1-0202. The government has certain rights in the invention.

TECHNICAL FIELD

This invention relates to methods and systems for generating a broadbandspectral continuum and continuous wave-generating systems utilizingsame.

BACKGROUND ART

Trends in DWDM sources include: increasing channel counts; narrowingchannel spacing; and the use of an array of single-frequency laserdiodes. However, the use of such arrays of laser diodes requires highcost for separate wavelength stabilization; inventory problems in caseof channel failure; and large space required for installation of sucharrays.

A spectrum-slicing scheme has been researched as a broadband wavelengthdivision multiplexing (WDM) source. The broad spectrum for slicing isobtained by various approaches including amplified spontaneous emission(ASE), a light-emitting diode, spectral broadening due to self-phasemodulation, a short-pulse mode-locked laser (MLL), and SC generation.Recently, the use of the longitudinal modes of an MLL pulse train as CWchannels has been discussed in spectrum-slicing multiple wavelengthsources. The technical advantage of the scheme is its precise spectralmode separation solely determined by the MLL pulse repetition rate. Togenerate the longitudinal modes, an MLL or an amplitude-modulateddistributed feedback laser is used.

However, there are various problems related to the spectrum-slicingconcept. The schemes utilizing the nonlinear spectral broadening of anoptically amplified pulse suffers from amplitude jitter due to four-wavemixing of ASE. In longitudinal mode slicing schemes, the narrow spectralextent (˜15 nm) and non-uniform power distribution among modes hindertheir application as broadband WDM sources. In addition, very highmodulation- or repetition-rates are required to satisfy current channelspacing standards.

U.S. Pat. No. 5,631,758 discloses a chirped-pulse multiple wavelengthtelecommunications scheme. Channel spacing is set by a modulator.

The Article by J. J. Veselka et al., entitled “A Multi Wavelength SourceHaving Precise Channel Spacing for WDM Systems”, IEEE Photon.,Technology Lett., Vol. 10, pp. 958-960, 1998 discloses anamplitude-modulated CW laser wherein channel spacing is set by therepetition rate of the amplitude modulation.

The article by H. Sanjoh et al., entitled “Multiwavelength Light SourceWith Precise Frequency Spacing Using A Mode-Locked Semiconductor LaserAnd An Arrayed Waveguide Grating Filter”, IEEE Photon., TechnologyLett., Vol. 9, pp. 818-820, 1997 discloses a mode-locked laser withchannel spacing actively determined by pulse repetition rate of thelaser and demultiplexer settings. Because the spectral range for CWchannels is not flat, external flattening is required.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a method and system forgenerating a broadband spectral continuum and a continuouswave-generating system utilizing same.

Another object of the present invention is to provide a method andsystem for generating a broadband spectral continuum and a continuouswave-generating system utilizing same wherein stabilization issimplified and the generation of multi-wavelengths is cost-effective.

In carrying out the above objects and other objects of the presentinvention, a method for generating a broadband spectral continuum isdisclosed. The method includes the steps of generating soliton pulses ata repetition rate greater than or equal to 1 Gb/s. The method furtherincludes compressing the higher-order soliton pulses in a temporaldomain through soliton-effect compression to obtain compressed solitonpulses having a spectrum. The method finally includes breaking up thecompressed soliton pulses to shape the spectrum of the compressedsoliton pulses through higher order dispersion effects and self-phasemodulation to obtain the broadband spectral continuum. The continuum hasa plurality of substantially uniform longitudinal modes each of which isa narrow band continuous wave in the time domain. The modes arespectrally spaced based on the repetition rate.

Preferably, the step of compressing includes the step of launching thehigher-order soliton pulse into a first end of an anomalous dispersionfiber including at least one pulse compression stage having a lengthbased on the order of the soliton pulse and a spectral shaping stage.The step of breaking up is performed adjacent a second end of theanomalous dispersion fiber in the spectral shaping stage. The spectralshaping stage is typically a dispersion-shifted optical fiber section ofthe anomalous dispersion fiber.

The step of compressing may be performed in multiple pulse compressionstages of the anomalous dispersion fiber.

The higher order dispersion effects typically include third orderdispersion effects. The shape of the broadband spectral continuum isbased primarily on the sign and magnitude of third order dispersion ofthe spectral shaping stage adjacent the second end. The shape of thebroadband spectral continuum is also based on pulse width of thecompressed soliton pulse immediately prior to the step of breaking up.The dispersion effects typically include second and third orderdispersion effects, and wherein the magnitude of the second orderdispersion and the magnitude of the third order dispersion normalized bythe pulse width have substantially the same order of magnitude adjacentthe second end of the anomalous dispersion fiber in the spectral shapingstage.

The spectral continuum is typically greater than 15 nm wide.

Further in carrying out the above objects and other objects of theinvention, a system is provided for generating broadband spectralcontinuum. The system includes a soliton pulse generator including asingle laser for generating soliton pulses at a repetition rate greaterthan or equal to 1 Gb/s. The system also includes an optical fiberincluding at least one pulse compression stage for compressing thesoliton pulses through soliton-effect compression to obtain compressedsoliton pulses having a spectrum. The optical fiber further includes aspectral shaping stage optically coupled to the at least one pulsecompression stage for breaking up the compressed soliton pulses adjacentan output end of the fiber to shape the spectrum of the compressedsoliton pulses through higher order dispersion effects and self-phasemodulation to obtain the broadband spectral continuum. The continuum hasa plurality of substantially uniform longitudinal mode each of which isa narrow band continuous wave in the time domain. The modes arespectrally spaced based on the repetition rate.

The at least one pulse compression stage of anomalous dispersion fibermay have a dispersion which is relatively constant therein.

The anomalous dispersion fiber may be a second pulse compression stageless than 100 and even less than 30 meters in length.

The dispersion effects typically include third order dispersions havinga sign and a magnitude. The third order dispersions may have either apositive sign or a negative sign.

The system may include a plurality of pulse compression stages ofanomalous dispersion fiber for compressing the soliton pulses throughsoliton-effect compression to obtain the compressed soliton pulses.

The length of the at least one pulse compression stage of anomalousdispersion fiber is typically based on a minimal width of the compressedsoliton pulses.

Yet still further in carrying out the above objects and other objects ofthe invention, a system for generating narrow band, continuous waves(CW) substantially simultaneously on multiple CW channels at multiplewavelengths and with channel spacing of at least 1 GHz is provided. Thesystem includes a soliton pulse generator including a single laser forgenerating soliton pulses at a repetition rate greater than or equal to1 Gb/s. The system also includes an optical fiber having at least onepulse compression stage and a spectral shaping stage. The at least onepulse compression stage receives the soliton pulses at a first end ofthe fiber and the spectral shaping stage generates a broadband spectralcontinuum within the fiber and provides the broadband spectral continuumat a second end of the fiber. The continuum has a plurality ofsubstantially uniform longitudinal modes each of which is a narrow bandcontinuous wave in the time domain. The modes are spectrally spacedbased on the repetition rate. The system also includes at least oneoptical filter coupled to the second end of the fiber for spectrallyslicing the longitudinal modes from the broadband spectral continuum toobtain the continuous waves.

The system may include a time domain multiplexer to adjust spacingbetween the longitudinal modes. The multiplexer may include a pulseinterleaver to increase channel spacing.

The single laser may be a single mode-locked laser such as a mode-lockederbium-doped fiber laser or a mode-locked semiconductor laser.

The single laser may be a single continuous-wave (CW) laser adapted tobe externally modulated.

The at least one optical filter may include a monochromator, an arrayedwave guide grating, a Fabry-Perot, a Mach-Zehnder, or fused-taperedcouplers.

The length of the spectral shaping stage may be less than 10 meters.

The spectral shaping stage may be a dispersion-shifted fiber.

The optical fiber may include a polarization preserving fiber.

The spectral continuum is typically greater than 15 nm wide.

The soliton pulse generator may include a polarization controller foraligning polarization of the soliton pulses with a polarizationeigenmode of the at least one stage.

The system may include a fiber amplifier or a semiconductor opticalamplifier for amplifying the soliton pulses. The fiber amplifier may bean erbium-doped fiber amplifier.

The optical fiber may include a high-nonlinearity (Hi-NL) fiber, apolarization preserving fiber, or a dispersion decreasing fiber.

The problems of narrow, non-uniform spectral extent are overcome withthe fiber-based SC generation and the amplitude jitter is overcome bymodifying the SC generation and a slicing scheme. As disclosed in theabove-noted application, SC in optical fiber results from theinteraction between self-phase modulation and third-order dispersion andforms a spectral region with ±0.5 dB flatness over >30 nm. Based onthis, one can obtain substantially uniform longitudinal modes over abroad spectral range by seeding the SC with a high repetition rate pulsesource. For example, a 140 nm SC at 10 GHz repetition rate can beobtained with an MLL and a dispersion shifted (DS) fiber.

To suppress the adverse effect of amplitude jitter on the transmissionperformance, the SC generation and slicing processes can be modified.First, the reduction of amplitude jitter due to noise-seeded four-wavemixing is considered herein. It is previously reported that during theSC generation in DS fiber, the ASE mapping through four-wave mixingleads to spectral coherence degradation increasing with propagationdistance. The random nature of ASE causes fluctuations in amplitude andfrequency.

To reduce the resultant deterioration in signal-to-noise ratio, thelength of DS fiber is minimized and the reduction in spectral broadeningis compensated by increasing the amplification in the present invention.

The influence of other longitudinal modes is also considered herein. Tosuppress the interference from adjacent modes that can be converted intoamplitude jitter in the spectrum-slicing process, the pulse repetitionrate is increased and, consequently, the mode spacing. With atime-domain multiplexing method, it is possible to increase them withoutchanging the source repetition rate.

After the time-domain multiplexing, each longitudinal mode is sliced asa CW channel. To check the degradation in CW amplitude stability, onecan measure RIN values at various wavelengths and compare them withsource laser RIN value. The RIN degradation is restrained within 7 dB/Hzin both anti-Stokes and Stokes regions as described herein. The CWchannels near the source wavelength experience higher levels ofdegradation.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a system for multi-wavelengths continuouswave generation (OA=Optical Amplifier; DSF=Dispersion Shifted Fiber;λo=Zero Dispersion Wavelength); an inset graph shows the filter functionof a monochromator taken with 0.05 nm (˜6 GHz) spectral resolution;

FIG. 2 is a graph which illustrates a profile of a supercontinuumgenerated with the system of FIG. 1; the profile has 48 nm spectralregion with ±0.5 dB flatness on the anti-Stokes side; on the Stokesside, the profile 30 nm wide spectral peak with equal or higher powerdensity; down-triangles indicate the wavelengths at which RINmeasurements are taken;

FIG. 3 is a graph illustrating a longitudinal mode structure in a flatspectral region after the pulse interleaver of FIG. 1; 40 GHz modespacing and >12 dB contrast are obtained at 6 GHz scanning resolution;

FIG. 4 is a graph of RIN measurement results; when compared to thesource laser, <7 dB/Hz degradation is obtained; and

FIG. 5 is a schematic diagram of a continuous wave-generating system ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A system of the present invention is shown schematically in FIG. 1. Thesystem includes a Pritel actively mode-locked fiber laser whichgenerates a hyperbolic-secant pulse train at 10 GHz repetition rate. Acenter wavelength is set at 1552 nm and the 3 dB bandwidth at 2.4 nm.With 1.1 psec pulsewidth, Δυ·Δτ=0.328. The laser output is amplified to800 mW average power by an SDL optical amplifier of the system.

A multi-section pulse compression scheme as described in the above-notedapplication is used to obtain the SC. In the system, three concatenatedsections of fiber are used. The first two sections of fiber areresponsible for soliton-effect pulse compression and spectralbroadening. In the third section of fiber, pulse breakup is induced inthe time-domain causing flattening of the broadened spectrum. The firstsection may be 6.8 meters of Corning SMF-28 and the second section maybe 15.7 meters of DS fiber with a zero-dispersion wavelength at 1492 nmand a dispersion slope of 0.039 ps/nm²-km. The third section may be 6.6meters of DS fiber with a zero-dispersion wavelength at 1546 nm and adispersion slope of 0.056 ps/nm²-km. The lengths of the first twosections are determined by cutting the fiber back until the optimumcompression point is encountered. The length of the third section is setto maximize the flatness of the SC. In both cases, additional cut-backsare done to verify that the first compression point is selected.

Spectrum of the generated SC is plotted in FIG. 2. It has 140 nm of 20dB bandwidth measured from the center peak. The spectrum has wider andflatter spectral broadening to the anti-Stokes side. This asymmetry inspectral shape results from the effect of the asymmetric third-orderdispersion. On the anti-Stokes side, one obtains ±0.5 dB flatness over a48 nm wide spectral region with a power density of +4.2 dBm/nm. On theStokes side, a 30 nm wide spectral peak is obtained above the powerdensity level of the anti-Stokes side. The power density of the Stokesside is 7.5 dBm/nm.

Time-domain multiplexing is preferably used to increase the longitudinalor spectral mode spacing. A pulse interleaver comprising of a 1-to-4splitter and delay lines is used to quadruple the repetition rate from10 GHz to 40 GHz. FIG. 4 shows a scan of modes obtained with an opticalspectrum analyzer. Across the flat spectral region and the spectralpeak, one obtains >12 dB peak-to-trough contrast. The contrast in thescan is limited by 0.05 nm (˜6 GHz) spectral resolution. The lengths ofthe delay lines are important since the imbalance in lengths results inincomplete cancellation of in-between modes.

For longitudinal mode slicing, a monochromator may be used as a filter.However, any optical filter with a narrow bandwidth can be used such asa Mach-Zehnder, an arrayed wave guide grating, a Fabry-Perot, orfused-tapered couplers. The filter bandwidth affects the channel outputpower as well as the interference from adjacent modes. Considering the40 GHz channel spacing in this case, the 20 dB bandwidth of the filteris set at 31 GHz. The filter function profile obtained with ASE input isshown in the inset of FIG. 1. At the output of the monochromator, a CWchannel is coupled to diagnostics for characterization. Due to thelosses in narrow-band spatial filtering and free-space coupling, theoutput power levels of the sliced longitudinal modes are around −41 dBmin the anti-Stokes region and near −39 dBm in the Stokes region.

The amplitude jitter of the CW channels can be characterized bymeasuring the RIN at various spectral points marked in FIG. 2. Theselected CW channel output is detected by a low-noise detector with 10MHz bandwidth. The detector output voltage is sampled 1500 times atevery quarter second. Based on the voltage samples, the RIN iscalculated according to the formula:

RIN=10*Log(ΔP/P _(ave))[dB/Hz]

where P_(ave) is the average power (i.e. the power contained in zero Hzfrequency component) and ΔP is the rms power spectral density of theintensity fluctuation. The detector input power is equalized to thelowest available level so that each measurement has the same average.The effect of minor average variation is corrected by normalization. Tocompensate the influence of detector noise, the average and rmsfluctuation of dark noise voltage are measured and subtracted in the RINcalculation. The RIN of the source laser is measured at the centerwavelength for a reference.

The RIN measurement results are shown in FIG. 4. The RIN of the sourcelaser is also marked. Except for one wavelength point at 1551 nm, RINvalues are uniformly distributed within ±0.7 dB/Hz range with an averagevalue at −98.9 dB/Hz. One can see that the whole process of SCgeneration and longitudinal mode slicing introduces less than 7 dB/HzRIN degradation in the anti-Stokes and Stokes side channels. It is clearin FIG. 4 that CW channels show worse performance near the source laserwavelength than on the anti-Stokes or Stokes sides. This RIN degradationcan be attributed to the rapid hopping of mode power in the source laseritself.

Based on the results noted above, one can use the system describedherein as a broadband WDM source. The block diagram in FIG. 5 shows abasic structure of the proposed source. An n GHz MLL is used for a seedpulse source. An optical amplifier enhances its pulse power for the SCgeneration at the next stage. The pulse interleaver increases thechannel spacing from n GHz to m GHz. At the last stage, channels aresliced with an optical filter. Each channel can be separately encodedand transmitted to form a transmitter.

In summary, a multi-wavelength CW generation system of the presentinvention is provided by spectrum-slicing of longitudinal modes infiber-based SC. Amplitude jitter in CW channels is suppressed bymodifying the SC generation and the spectrum-slicing scheme of theabove-noted application. A short-fiber SC generation scheme is providedand a pulse interleaver is added to reduce the influence of adjacentmodes.

From a 140 nm SC seeded with 10 GHz pulse train, CW channels are slicedwith 40 GHz channel spacing by a monochromator. The RIN is measured atvarious wavelengths to check the amplitude jitter. Except for the modesnear the source laser wavelength, uniform distribution of RIN valueswithin 1.4 dB/Hz range is observed with −98.9 dB/Hz average value. Thisresult indicates that less than 7 dB/Hz RIN degradation occurs duringthe whole process of SC generation and spectrum-slicing.

This scheme can be utilized as a broadband WDM by increasing the numberof uniform channels. When the source is between 100-200 channels and thedata rate is 622 Mb/s˜10 Gb/s the source can be utilized in a number ofdifferent applications. For example, the source can be utilized in along-haul transmission system (i.e. 100 channels*10 Gb/s=1 Tb/stransmission). The source can also be used in a multi-channelMetropolitan Area Network (MAN) or a Local Area Network (LAN) or in amultiple wavelength testing system.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A method for generating a broadband spectralcontinuum, the method comprising: generating pulses at a repetition rategreater than or equal to 1 Gb/s; compressing the pulses in a temporaldomain to obtain compressed pulses having a spectrum; and shaping thespectrum of the compressed pulses through higher order dispersioneffects and self-phase modulation to obtain the broadband spectralcontinuum, the continuum having a plurality of substantially uniformlongitudinal modes each of which is a narrow band continuous wave in thetime domain and wherein the modes are spectrally spaced based on therepetition rate.
 2. A system for generating broadband spectralcontinuum, the system comprising: a pulse generator including a laserfor generating pulses at a repetition rate greater than or equal to 1Gb/s; and an optical fiber including at least one pulse compressionstage for compressing the pulses through non-linear effects to obtaincompressed pulses having a spectrum, the optical fiber further includinga spectral shaping stage optically coupled to the at least one pulsecompression stage adjacent an output end of the fiber to shape thespectrum of the compressed pulses through higher order dispersioneffects and self-phase modulation to obtain the broadband spectralcontinuum, continuum having a plurality of substantially uniformlongitudinal modes each of which is a narrow band continuous wave in thetime domain and wherein the modes are spectrally spaced on therepetition rate.
 3. A system for generating narrow band, continuouswaves (CW) substantially simultaneously on multiple CW channels atmultiple wavelengths and with channel spacing of at least 1 Ghz, thesystem comprising: a pulse generator including a single laser forgenerating pulses at a repetition rate greater than or equal to 1 Gb/s;an optical fiber including at least one pulse compression stage and aspectral shaping stage, the at least one pulse compression stagereceiving the pulses at a first end of the fiber and the spectralshaping stage generating a broadband spectral continuum within the fiberand providing the broadband spectral continuum at a second end of thefiber, the continuum having a plurality of substantially uniformlongitudinal modes each of which is a narrow band continuous wave in thetime domain and wherein the modes are spectrally spaced based on therepetition rate; and at least one optical filter coupled to the secondend of the fiber for spectrally-slicing the longitudinal modes from thebroadband spectral continuum to obtain the continuous waves.
 4. A methodfor generating a broadband spectral continuum, the method comprising:generating pulses at a repetition rate greater than or equal to 1 Gb/s;compressing the pulses in a temporal domain to obtain compressed pulseshaving a spectrum; and shaping the spectrum of the compressed pulses toobtain the broadband spectral continuum, the continuum having 100 ormore substantially uniform longitudinal modes each of which is a narrowband continuous wave in the time domain and wherein the modes arespectrally spaced based on the repetition rate.
 5. A method forgenerating a broadband spectral continuum as a transmitter, the methodcomprising: generating pulses at a repetition rate greater than or equalto 1 Gb/s; compressing the pulses in a temporal domain to obtaincompressed pulses having a spectrum; shaping the spectrum of thecompressed pulses to obtain the broadband spectral continuum, thecontinuum having a plurality of substantially uniform longitudinal modeseach of which is a narrow band continuous wave in the time domain andwherein the modes are spectrally spaced based on the repetition rate;and coupling the continuum to a length of transmission fiber in along-haul transmission system.
 6. A method for generating a broadbandspectral continuum as a transmitter, the method comprising: generatingpulses at a repetition rate greater than or equal to 1 Gb/s; compressingthe pulses in a temporal domain to obtain compressed pulses having aspectrum; shaping the spectrum of the compressed pulses to obtain thebroadband spectral continuum, the continuum having a plurality ofsubstantially uniform longitudinal modes each of which is a narrow bandcontinuos wave in the time domain and wherein the modes are spectrallyspaced based on the repetition rate; and coupling the continuum to alength of transmission fiber in a metropolitan area network.
 7. A methodfor generating a broadband spectral continuum as a transmitter, themethod comprising: generating pulses at a repetition rate greater thanor equal to 1 Gb/s; compressing the pulses in a temporal domain toobtain compressed pulses having a spectrum; shaping the spectrum of thecompressed pulses to obtain the broadband spectral continuum, thecontinuum having a plurality of substantially uniform longitudinal modeseach of which is a narrow band continuous wave in the time domain andwherein the modes are spectrally spaced based on the repetition rate;and coupling the continuum to a length of transmission fiber in a localarea network.
 8. A method for generating a broadband spectral continuumas a transmitter, the method comprising: generating pulses at arepetition rate greater than or equal to 1 Gb/s; compressing the pulsesin a temporal domain to obtain compressed pulses having a spectrum;shaping the spectrum of the compressed pulses to obtain the broadbandspectral continuum, the continuum having a plurality of substantiallyuniform longitudinal modes each of which is a narrow band continuouswave in the time domain and wherein the modes are spectrally spacedbased on the repetition rate; and coupling the continuum to a multiplewavelength testing system.